Friday, May 16, 2014

Hurricanes and typhoons are migrating from the tropics toward the North and South poles, a new study finds.

In the past 30 years, the total number of storms has remained about the
same in the tropics, said lead study author Jim Kossin, a climate
scientist with the National Oceanic and Atmospheric Administration's
National Climatic Data Center.

What are hurricanes, typhoons and tropical cyclones? How do they form?

(MetOffice)

What has changed, however, is the number of successful storm births.
The new study found that tropical storms
don't peak in the tropics as often as they did 30 years ago. Instead,
more and more storms are reaching their maximum strength at higher
latitudes, according to the report, published May 14th, in the
journal Nature.

"The tropics are becoming less hospitable for tropical cyclones, and
the higher latitudes are becoming less hostile," Kossin told Live
Science's Our Amazing Planet.
Tropical cyclones (the broad name for hurricanes, typhoons and tropical
storms) spin up over and over in the same regions — a group of storm
nurseries ringing the tropics — because of favorable wind patterns and
ocean temperatures.

Typhoon Francisco and Super Typhoon Lekima on October 23, 2013

as they tracked northwestward toward China and Japan.

(Credit: Tim Olander and Rick Kohrs, SSEC/CIMSS/UW-Madison,

based on Japan Meteorological Agency data.)

Storm nurseries stir

Kossin and his co-authors think a simultaneous expansion in the
planet's tropical belts underlies the overall change in storm intensity.
The tropics have widened
by about a degree in latitude each decade since 1979, according to
separate studies by other research groups.
The expansion also could have
pushed the ideal storm-forming regions toward the North and South
poles.

"There is certainly compelling evidence the two are linked, but we're
not sure exactly how — that's what we want to find out," Kossin said.
"This is a link that needs to be examined."
The expansion of the tropics has been linked to global warming and ozone loss.
But scientists still hotly debate the impact of global warming on hurricanes.
Storms could become more or less frequent, more intense or a combination of these changes, researchers say.
"This study establishes another link between global climate change and
global tropical cyclone activity," said Hamish Ramsay, a climate
scientist at Monash University in Australia who was not involved in the
research.
"It also raises a number of new questions, though."
The poleward trek doesn't necessarily mean that ferocious storms will
be hitting the Atlantic coastline more often.
As climate changes,
fluctuating wind patterns could cause tropical storms to move toward or
away from coastlines, for instance.
And the study didn't examine
landfall, where storms do the most damage.
Another confounding factor: The Atlantic Ocean storm nursery did not
move north in the past 30 years, the researchers reported.
Kossin said
he suspects that regional effects in the Atlantic, such as aerosol
pollution (tiny airborne particles), could be offsetting the overall
tropical widening.

Color-enhanced infrared satellite image of Typhoon Usagi as it moved northwestward toward Hong Kong while explosively intensifying to a Category-5 storm.

Image: NOAA/Cooperative Institute for meteorological satellite studies, University of Wisconsin-Madison

Heading north

By tracking where tropical cyclones hit at their strongest point,
called peak intensity, the scientists discovered that storms are heading
north and south.
This method avoids problems with comparing storms
between different oceans, Kossin said. Determining peak intensity is
relatively consistent among different storm-tracking centers, he said.
Other criteria, such as when a tropical storm tips into hurricane
strength, can vary from center to center, making comparisons difficult.

The push poleward averaged about 33 miles (53 kilometers) per decade in
the Northern Hemisphere and 38 miles (61 km) per decade in the Southern
Hemisphere — a total shift of about 1 degree latitude per decade.
But
some oceans saw a greater change than others.
The biggest moves occurred
in the Pacific Ocean and South Indian Ocean, but the peak intensity of Atlantic hurricanes and storms in the North Indian Ocean showed almost no change.